Hope this isn't too late.
All isofix seats will either use a leg or a top tether. Unfortunately top tether isn't particularly great for rear facing seats (the way it works here) as it's too flimsy - Australian style with a Y shaped tether is fine, but that tends to impede the spin.
Therefore the only rear facing options with isofix and tether are either limited (e.g. Maxi Cosi Axissfix, rear facing up to 87cm, FF to 105cm) or they are cheaper/generic type brands, either then sold under a bigger brand like Cosatto/Kinderkraft/My Babiie or just one of these little known ones you find on amazon, ebay etc. The problem with the generic seats is they tend to only allow you to use the recline position when rear facing which gets annoying for older children who want to sit up. In some models the top tether placement doesn't let you spin the seat and/or they need the belt around the back when rear facing which again doesn't let you use the spin. And they tend not to do very well for crash safety.
Top tether works well for forward facing, but you might not want to forward face yet.
So then you're left with looking at belted seats like Joie Stages, which has an annoying belt path, Britax Two Way Elite which braces on the front seat instead of using a leg and is quite a basic, old-fashioned (like 1990s) seat.
Or you look at ways of integrating the leg with the box. Which isn't as simple as being a straight no.
If your Peugeot is 2018 ir newer, the box lids are reinforced and you can put a leg on them.
If it's older, you may be allowed to put a leg to the bottom of the box.
My 3008 has a confusing bit in the manual about the middle seat - it's unclear whether you can put a leg there, but may be possible.
One last option is trial and error with different seats to see if you can find one that misses the box entirely.
If you do find a way to work with the box, then you have the option of spinning isofix seats to 18kg, belted ERF seats to 25kg (these tend to be more flexible in fitting to avoid boxes) or Axkid One which is isofix up to 23kg. If your child is big for their age, the seats with an 18/19kg limit will likely be too small for them before they reach 4 years old, so you may want to look at the larger capacity options even though isofix is limited with these seats.
Just rule out anything Britax as they say a flat no to storage box + leg combo regardless of anything else.