Not that stretchy to be honest...the Rezo 30 (also at the same price from the same retailer) is 35mm and is sure to fit snugly, if the dimensions you are matching are correct..
As for sliplock being the belt attachment, i don't think it is. Sliplock is the accessory attachment system. TBH I can't remember whether mine had a belt attachment.
I also used to own a Lowepro DRES-8 and that certainly did. Might be worth a quick email to Ffordes to ask, or give them a quick ring, or someone might come along to confirm in the meantime.
Sorry I can't give definitive answer