Wheelchair carriage assistance

I have a friend who reach out to me.
He needed my help to design something that could help him transport heavier things when he was at work. 
So togheter we tested this design and it came to an success.

You have to print this with a lot of walls to maximize the carriage weight. He told me it could hold up to 35kg.
We have tested it with PLA and PETG and both worked fine but for maximum strenght, you should maybe go with stronger filament types. It's up to you.

* There is no guarantee the fasteners works with every wheelchair
*Please note: I did this when I hade more limited CAD skills. If there is something of or anything just let me know and I will try to help improve it.

Just attach it with nuts, washer and bolts. The holes isn't perfect but it works.

The plan was to "massproduce" these and sell them to people who would need them but insteed I took the decision to put them out here for free. If people finds this and need this then they should have it.