well i have two of the little babies , both just two and almost the same age, they keep each other company most of the time , when they are not fighting that is. We live on 7acres, so my babies have plenty of room to run and play.and swim. which i think relieves there boredom and distructiveness. My husband would never allow my dogs in side. But the odd time they sneak in, yes they run straight for trouble. so i could imagine what you are going through. I must admit if i have been busy with work and havnt given them the ut most attention, they begin to start gettn in to things outside and makn a mess, shredding plants, chasing chickns, rounding up goats together and worst of all getn into my husbands shed and pulling out things and bringn them up to the house. Rope is there favourite. so now we have given them a long bit of there own to play with, they spend hours playn with it. I found the best way to relieve this distructive boredom is give them all your spare attention, allways have things to amuze them and place these items in there area, and i always give my dogs a bone or pigs ear each to occupy them(seperately of course or they fight) i dont crate mine, but at night they toad hunt and chase snakes so at 7pm they go to bed with a treat in their snug dog trailer which is really big . When im at work or go out, they have big day runs, one each side by side, i cant put them together anymore as they fight. In these dog runs i have slinged up stockings from one side to the other, any many dangly toys hang from it, also i give them a piece of rope each and a bone. sometimes i put something different in there like a empty box , something that makes them curious. All of this i found has helped me alot, and i cant really say i have destructive boxers , my last boxer was , but i learnt alot from her. So Consider trying something like , i beilieve theres usually a reason a boxer does a naughty thing like yours did.
goodluck