Basic Explanation 

This is a simple holder that is made to store prototype boards that can drop onto a Gridfinity plate. It takes up two slots on the baseplate and is not stackable. There are no holes at the bottom for magnets.
 

This holder can hold:

• 8cm x 2cm boards (holds 3) (First slot)
• 7cm x 3cm boards (holds 4) (Second Slot)
• 6cm x 4cm boards (holds 3) (Third slot)
• 7cm x 5cm boards (holds 3) (Fourth slot)

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My print settings:

I used the Bambu Lab A1 to print this and sliced using Bambu Studio. 

• I used 15% infill.  
• I used 2 Wall Loops. Kept this at the default setting.
• I did not enable supports.

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Extra info about the holder:

• The holder is sloped (shown in image 3) so the shorter ones near the front can easily be picked up whilst the larger ones near the back have enough wall size to keep it secure. 
• I bought these prototype boards from Amazon and are made by AZdelivery. This holder may fit other brands.
• Feel free to edit the model in whatever way you like.

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Link to the specific boards I have:

https://www.amazon.co.uk/gp/product/B078HV79XX/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

(AZDelivery PCB Prototyping Board Perforated Cards)