Use dpd most weeks
Had a problem with Italy last week but that was down to us forgetting the invoice
Get b&q or similar to cut some 15 or 18mm ply or mdf. Just tell them the sizes. Then totally overpack inside it. We had to make a largish box on Monday for a 40kg item. Is boring but worth it
I have used Interparcel quite a few times.
Pack to more than you think it needs then pack it some more.
If you are not confident about dropping the package from 1m onto a concrete floor landing on a corner then you haven’t packed it well enough
Check with your courier if they’ll accept a plywood box. I remember having to wrap a ply box in order for the courier to accept it. No idea why. Some jobsworth cuntery.
I sent an expensive mains cable to northern Italy two weeks ago with UPS via Interparcel. No problems at all. I could track it and they sent me texts when out for delivery and on delivery.