I once made a mini table ( like footstool size ) – 4 legs and a flat top – from plywood. Then under I screwed 4 hooks to hang toys from.
It worked great until I got Boston who hates being under anything as his eye sight is very poor.

Another option is to get a foot stool and cover the top with fleece – leave strips of fleece thick enough to knot toys into on either side ( make sure the toys on one side weigh the same as toys on other so it balances )

Or a children’s table could work ( if safe material ) and you could screw on the underside large curtain hooks to hang toys from

My buns don’t have cages but I have a pen panel that I’m using to block the radiator so I just hang stuff on the panel.

The low table would work well. If you look up Activity Zone for rabbits, you’ll see example of that. Some members here were using NIC grids to create Fleece Forests  but you could use them to add hanging toys too. Q8 didn’t have grids so she glue fleece strips to underside of a chair.

But you cable tie a grid under a low chair and use it to hang stuff up. 

I see the Ikea doll bed in the background there. You could even hang toys on the bed ends. : )

If making something entirely out of grids, I think you would have to make a full cube (cable-tied) or 5-sided one at least. Or make 2 cubes then cable tie together for extra strength. Especially since you have a larger breed bun who is growing! You could fix a board, cardboard or a hay mat to the top so he can jump on top. 

Or a wood fruit crate tipped on it’s side might work. You could fix some toys to part against the floor also. (Knagglig box at Ikea is untreated pine).

Cat tower with hidey box at bottom? Fix some toys to the side?

ETA: Clothes drying racks too! You can drap blankets over to to create a tunnel/cubby and hang things on the lower bars. So long as not easily toppled.

That should work well.

If you like the fleece forest idea, this is an older thread pinknfwuffy created: https://binkybunny.com/FORUM/tabid/54/aft/148130/Default.aspx

She also posted this in an older Pic thread. 

Just saw this DIY pigloo/fleece forest on a guinea pig forum by username missbounce. She bought the children’s step-stools at a local Tesco but I assume they could also be found for cheap at most dollar stores, Walmart, etc. Attached some fleece with velcro so she could remove the fabric to wash. So cute! I think it’s a fun idea for those who like igloos but worry about ventilation or them being too small for most of our buns.