There are direct 2 reservation-free possibilities:

There are also still many options available for 20€, and one for 10€ (Mulhouse dep. 16:56). You can check that via Belgian railways. You can book there if you have a pass cover number and with 4€ booking fee. If you're in France or Switzerland, just go to the ticket office.

The Interrail reservations module has a long-standing bug that it doesn't show the 20€ reservations.

Thanks a lot for the reply, unfortunately the free option are so much longer than the tgv. And now it’s terrible i can’t find anything valid, only on the sncb website but that one requires a pass cover number and i don’t have a paper pass. This whole systems looks so stupid and complicated for a pass i’ve already paid for. Thanks for your answer and i will continue looking (only one i’ve found is a 37 eur reservations but that’s almost a full fare i’m paying for. 

To get your pass cover number you can contact @Nanja 

IF you are in FR NOW you can also do it easy (and without the long wait at station-counter) at any SNCF LONG distance machine-though you have to push a long row of buttons-start with ticket, then alter choose ´reduction´ and then find Inter or EUrail. Payment only with back/cr-cd.

Yes-again you see why these recently built superfast trains cost more-they shorten the travel time with over half. The ślow TER times/trains are how it used to be untill these opened.