Unfortunately we haven't got a list of Modular oven sellers/manufacturers on the forum for you to choose from. This is due to problems on the last forum with commercial companies filling the open forum promoting their products.
Any member can however, give their own opinion on which modular oven they think is the best.
As to your question......to keep the weight down and make this economical a cheaper model of a modular oven may be a good option.
The best ovens for a trailer in my opinion are the ovens from Dingley Dell. Although he builds them on trailers, he does also sell them on their own. His ovens are about £1500, so if this was outside your price range you could go for one of the Portuguese ovens. They start off at about £550 but you would have to add insulation and waterproofing yourself. You may also need a little under hearth insulation. The 2" thick thermolite briquettes (laid flat) would be ideal for a trailer. If you go down this route try to get one with the chimney positioned towards the front.
You could go down the route of casting one yourself but the challenge there is keeping the weight down.
I think "Slowfood" is already on his second Mobile Wood-fired oven build so he must have learned a lot buy building one.
All the best with your project and I hope you get some more feedback on this.
Thanks Canny, Well every project is a learning curve, At the moment I am converting a VW t4 into an escape pod stealth camper. With a very low budget, It's coming on well but very slowly, fully insulated to keep me warm on the coldest of nights, 50mm kingspan, frost blanket and 6mm plywood, great project, However now I have lost my job I intend to resume building moulds for ovens when required, Good luck