Need small speargun

Hi spearfishers. I'll likely be regarded as non-serious and I suppose I am, but here's my situation. I was exiled by Covid in March 2020, luckily finding myself living next to a big lake I can swim in every day of the year, which I've done the whole time. I never saw any normal-sized fish until just recently, when I found large schools on the other side of an inlet narrow enough to swim across. Remembering one summer in Vermont when I speared a small fish in a pond with a little homemade speargun and took it home and fried it for dinner, I thought I could do the same thing here and ordered a Mares Bandit Sling Speargun from Amazon. I wanted to get the smallest one available, the 35 cm, but ChatGPT advised me to get the 45 cm instead and so I did that. I picked it up the other day and was immediately dismayed, because even though it's one of the smaller sizes it's way too big and heavy for me to be able to comfortably swim and shoot with it. I googled on "one-handed speargun" and there was exactly one find, that one being a fictional weapon for video games. I wanted something like a pistol, but apparently they're all like rifles. Since the one I ordered had to come from the States and wait for me to pick it up in a nearby town I don't go to often, I'll probably be too late for the 30-day return for a refund, which would mean I'll have a store credit with Mares. Are they going to be able to fix me up with something I can practically use as an unequipped lake swimmer (used trade-in would be okay)? If not, does anyone here know of something that would be appropriate for me? It seems strange that I had a light little gadget that did the job back in Vermont, but I haven't been able to find anything similar that's commercially available now. Thanks.