need a fin...

I would like to give primbee a go but only have older boards - Mistral screamer 2 and Naish Titan .

Screamer is - 278 x 55 x 103L with 34cm PB fin
Titan is - 268 x 66 x 135L. 38cm Freeride PB fin

I believe i would need a weedy for primbee, is that right? If so is there one fin i could use between both boards?

I nearly fell over when i looked at the price of new weedies.!!

Thanks