This is a simple yet versatile parametric 90 degree bracket.  This bracket can be re-sized for virtually any need via the OpenSCAD parameters.  The bracket can also be set to 3 different styles; Normal Bevel, Low Poly, and Extra Low Poly.

If you're not familiar and/or hate using OpenSCAD, you can use MakerWorld's Customizer here (https://makerworld.com/en/models/726914#profileId-674133) to create any custom size brackets you may want. Prusa, this it really something you need to add to your site!!!!

I have pre-generated STLs for a set of imperial sizes from 1x1 to 8x8 inches that use #4, #6, or #8 screws.  See the table below for the sizes and screw placements/measurements.  I will be adding more pre-generated metric STLs over the next week.

If you want to create a custom size, I have uploaded the OpenSCAD file that you can use to change the parameters. You can modify the height, length, width, screw hole placement and size, screw head size, and poly style, and more. I have also uploaded the .json file that contains the settings for the pre-generated STLs listed below.  
NOTE: The OpenSCAD file is not perfect :-/ Sorry, but it take too much time to be perfect.  If you make a setting too big or too small you might get something that doesn't even look like a bracket.  However, you can use the settings in the .json file as a starting point. I needed to upload this as a txt file, so to use it, you'll need to rename it to 90DegreeBracket.json if you want the “Parameters” tab to display in OpenSCAD.  
NOTE #2: Yes, I know the normal bevel style takes a looooooonnnnnnnggg time to render(like 5 minute sometimes) :-(. Sadly OpenSCAD has some mesh issues sometimes, so I needed to use a very large polygon count to get everything to line up correctly :-/.  I'll try to fix the rendering time later after I make a few other enhancements.

Pre-Generated STLs

Imperial

Bracket Size

Bracket Width

Styles

Screw Size

Screw 1 Placement

Screw 2 Placement

1x1 Inch

¾ inch

Normal Bevel, Low Poly, Extra Low Poly

#4 or #6

5/8 inch

None

2x2 Inch

¾ inch

Normal Bevel, Low Poly, Extra Low Poly

#4 or #6

5/8 inch

1 5/8 inch

3x3 Inch

¾ inch

Normal Bevel, Low Poly, Extra Low Poly

#4 or #6

7/8 inch

2 ½ inch

3x6 Inch

¾ inch

Normal Bevel, Low Poly, Extra Low Poly

#4 or #6

3 inch side: 
7/8 inch

6 inch side: 
1 ¾ inch

3 inch side: 
2 ½ inch

6 inch side: 
5 inch

4x4 Inch

¾ inch

Normal Bevel, Low Poly, Extra Low Poly

#6

1 inch

3 3/8 inch

6x6 Inch

1 inch

Normal Bevel, Low Poly, Extra Low Poly

#6 or #8

1 ½ inch

5 inch

6x8 Inch

1 inch

Normal Bevel, Low Poly, Extra Low Poly

#6 or #8

6 inch side: 
1 ½ inch

8 inch side: 
2 inch

6 inch side: 
5 ¼ inch

8 inch side: 
7 inch

8x8 Inch

1 inch

Normal Bevel, Low Poly, Extra Low Poly

 #8

2 inch

7 inch

Metric

Bracket Size

Bracket Width

Styles

Screw Size

Screw 1 Placement

Screw 2 Placement

20x20 mm

20 mm

Normal Bevel, Low Poly, Extra Low Poly

M3 or M4

12 mm

None

20x40 mm

20mm

Normal Bevel, Low Poly, Extra Low Poly

M3 or M4

20 mm side: 
12 mm

40 mm side: 
24 mm

None

30x30 mm

20 mm

Normal Bevel, Low Poly, Extra Low Poly

M3 or M4

10mm

22 mm

40x40 mm

20 mm

Normal Bevel, Low Poly, Extra Low Poly

M3 or M4

15 mm

32 mm

40x60 mm

20 mm

Normal Bevel, Low Poly, Extra Low Poly

M3 or M4

40 mm side: 
14 mm

60 mm side: 
21 mm

40 mm side: 
30 mm

60 mm side: 
45 mm

60x60 mm

20 mm

Normal Bevel, Low Poly, Extra Low Poly

M3 or M4

20 mm

50 mm

If you find anything wrong with the tables above, please let me know so I can fix it.  Thanks!

Printing

I recommend printing these flat with a few pain on supports inside the screw access slot in the angle support. See images for an example.

Otherwise, use your discretion on fill and perimeters depending on how strong you need them to be.

Change Log

2024-09-22:

• Added 20x40, 40x60, and 60x60 mm brackets.
• Fix a small issue that was leaving a small amount of material behind after the screw hole access cutout was completed.   Was only and issue in a small amount of situations… :-/  But I should have caught it sooner…
• Updated the 90DegreeBracket.json settings file.  Which remember is uploaded as 90DegreeBracket.txt since Printables doesn't allow *.json uploads.

2024-09-15:

• Noticed the #8 screw holes were a little undersized.  Made them a little larger.
• Updated the 90DegreeBracket.json settings file.  Which remember is uploaded as 90DegreeBracket.txt since Printables doesn't allow *.json uploads.

2024-09-14:

• Added 20x20, 30x30, 40x40 mm STLs. 
• Fixed a small issue in the scad file where it was possible to create an un-renderable STL due to a cutout not being wide enough and leaving a “skin/layer” behind.
• Updated the 90DegreeBracket.json settings file.  Which remember is uploaded as 90DegreeBracket.txt since Printables doesn't allow *.json uploads.