I applaud you for your attempts to build your piggies a large cage because they need a large, flat surface with lots of ventilation for their health and wellbeing.
If may offer you some constructive criticism from my own hard learned lesson... Wood is oftentimes very difficult to keep adequately clean even with a covering over it (such as tile or linoleum) because moisture inevitably gets to it which causes issues down the road. Sure, it looks great and works well at the start, but all you have to do is deal with one pig with a fungal infection, urine wearing construction away or an especially busy pig who likes to gnaw away at the wood (which is a natural thing for pigs to do) to have a really frustrating experience. Any exposed wood is impossible to clean adequately to solve some issues and oftentimes just creates a messy, odorous situation for you and your pigs.
I have found the c&c cages to be the easiest to clean, healthiest for my pigs (lots of space, ventilation, and easy to disinfect when necessary) and the most versatile...I can add to it or change the configuration whenever I want at an affordable price! I couldn't believe how much better it was until I switched to one.
I hope that you are also planning on this structure being indoors. Many times those choosing wood are planning an outdoor cage which is not usually in the best interest of their pigs' health. (I am sure you have already read about their sensitivity to certain humidity and temperature ranges)
Just consider all your options of cage materials since I am sure you plan to put a lot of effort into this project.
And remember, the people on this site are very knowledgeable and are a wonderful help because they have an immense amount of experience with guinea pigs. Some have made their own mistakes and learned the hard way, so they want to share with others for the benefit of their guinea pigs. They really do know what they are talking about and are reliable source of information.