If your goal is to carry bulky but light items, another DIY way would be to take an adapter for a known attachment system and screw a plywood plate on it — or you can also go more custom.

The picture below shows the example for a RackTime Snap-It rack, but there are other systems (for example, KlickFix — if someone knows another one, just write it in the comments).

The advantage of this approach is that it's super easy to remove your attachment when you don't need it. But you may need to change your rack to have something with the system you've chosen.

Note that there are many available attachments for these systems, maybe there's one that could fit your needs.

If your goal is to carry reasonably long but light items, another DIY way would be to take an adapter for a known attachment system and screw a plywood plate on it — or you can also go more custom.

The picture below shows the example for a RackTime Snap-It rack, but there are other systems (for example, KlickFix — if someone knows another one, just write it in the comments).

The advantage of this approach is that it's super easy to remove your attachment when you don't need it. But you may need to change your rack to have something with the system you've chosen.

Note that there are many available attachments for these systems, maybe there's one that could fit your needs.

But for very long or heavy items, there's a risk of going too far off design specifications. A (longtail) cargo bike might be a better solution.

If your goal is to carry bulky but light items, another DIY way would be to take an adapter for a known attachment system and screw a plywood plate on it — or you can also go more custom. The picture below shows the example for a RackTime Snap-It rack, but there are other systems (for example, KlickFix — if someone knows another one, just write it in the comments). For this approach, you may need to change your rack to have something with the system you've chosen.

The advantage of this approach is that it's super easy to remove your attachment when you don't need it.

Note that there are many attachments commercially available for these systems, maybe there's one that could fit your needs.

If your goal is to carry bulky but light items, another DIY way would be to take an adapter for a known attachment system and screw a plywood plate on it — or you can also go more custom. 

The picture below shows the example for a RackTime Snap-It rack, but there are other systems (for example, KlickFix — if someone knows another one, just write it in the comments).

The advantage of this approach is that it's super easy to remove your attachment when you don't need it. But you may need to change your rack to have something with the system you've chosen.

Note that there are many available attachments for these systems, maybe there's one that could fit your needs.