Replacement of Bubble Seal in Casement Window

I attach photos of the cross sections of old and recently bought seals. They are very similar dimensionally. My problem is that the new replacement seal does not fit in the kerf easily. The old seal fits well and this may be a consequence of its age. I tried using a normal screen roller without success. First then assuming the new seal is OK, is there a trick or tool in your toolbox that would help installation? Second if you think there is a better solution for this situation [would like to use the kerf regardless] please advise. Width of old seal bulb is 3/8" squished,base on which the bulb is attached is 3/16" and the depth of kerf insert is 1/4". For sure the fins on the kerf insert are very pliable on the old seal but thicker and stiff on the new seal.