Hi bud I love the design that you created, any chance of me getting any tips on how to build it cheers
Welcome to the Bunnings Workshop community. We're pleased to have you join us and look forward to reading more about your projects and plans for around the house and garden.
I would assume that @MartyH actually shared this as inspiration and that unfortunately it's not his project. Hopefully he and other helpful Workshop members like @r23on and @Brad will have some ideas in how you might build something similar.
Please let me know if you ever need a hand getting the most from the site.
I would make a frame cover it with blueboard and then render it, making the round corner will take some skill, a nice bend can be made on a table saw by making a series of cuts that do not go all the way through the timber and the closer the slots are together the tighter the radius can be. Epoxy will lock it into the shape you want. Not sure how tight the bends can be with blueboard, cheating would be using a bendable ply or slot cutting.
Make a mold and fill with concrete and it will need a good foundation to hold that weight and reinforcing rod or mesh used. Formply is now the usual material of choice for forming concrete it is not so great on curves, bendy ply may be the answer with a silicone coating to aid release. BBQ cut out will be a test of skill, I would prefer to be on the tight side and clean it up with an angle grinder than try to fill gaps. A concrete vibrator would be handy to help prevent voids in your cast.
Like Brad has suggested a good foundation is the key to the project. The construction I would be thinking more of is to use 80 x 80mm x 5mm RHS to construct the main frame work well anchored in the footings. The cut out section for the BBQ 50mm x 50mm x 3mm. Then as suggested blue board for covering etc