The onbag pattern is free to use I think, so if you're okay at sewing you could give it a go! https://onbag.co.uk/diy-pattern That way you'd only have the materials to buy.
I normally just take a big-ish handbag that I either have over my shoulder and it sits slightly forward of where it would normally because the baby pushes it forwards or I carry it in my hand like a shopping bag. Or I just take a shopping bag, or if it's a shorter journey I use small wetbags, the ones with the poppered strap and just clip them onto the carrier somewhere. One of mine matches the R&R so I use that most. I can just fit my purse, phone, lipbalm, tissues in one and if I take another, I'll put a nappy in there too. That would only really work for short trips though!
You could also look at those granny trolleys, I think Ikea sells them.
I have learned to pack light now, sort of. Analyse everything you normally take e.g. do you really need a full purse? Why not take a debit card and a tenner instead of everything? Clubcards can stay at home! How many nappies do you really need? Maybe take a couple of flat nappies and a wrap or two instead of three full pockets. I don't take drinks or snacks out now when I need to travel light - they can be bought if needed. Toys can be improvised or take something very small but fun.