I found this glitch by accident while playing as Wario on a custom stage. Here's how it's done.

1.) First, build a custom stage in the Stage Builder. The custom stage MUST have the following features:

• One Stage Builder part in which Wario can fit in to EXACTLY one space ABOVE the ground. Ex. One medium to large size single block. Ex. One medium to large size structure (depends on the type of structure you're using and/or with what theme/background you're using for your custom stage.)
• One moving platform (the one that moves left to right and is located directly below the structures section of the building parts menu) placed DIRECTLY UNDER THE OBJECT IN WHICH YOU WANT WARIO TO GO THROUGH!!! Or the thin steady platform which is in the first section of the build menu's parts placed directly under the object that you want Wario to walk through.
• A long flat solid ground of regular blocks. *Large enough for Wario's bike to drive safely on it without crashing into anything or falling off the stage.

*(I know my descriptions of the objects required to do this glitch aren't all that great, so if you have any questions whatsoever, please feel free to ask in the comments below!)*

2.) Now, go to a regular brawl and have your character play as Wario and your opponent play as anyone and go to the stage selection screen and go to your custom stages and select the custom stage that you just made. Now start the brawl!

3.) Once the brawl is started, as Wario, go to the object that you want Wario to go through. Stand a slight distance to the side of the object and get on Wario's bike and go up to the object as slow as possible.

4.) Finally, once directly under the object and still on Wario's bike which is moving very slowly, pop a wheelie and hold it. CAUTION: Do not have the bike's rear tire touch the ground! If this happens you will fall off the bike and not go through the object and you will have to try again!!

If done correctly, what should happen is that while doing a wheelie, Wario's bike will crash into the object directly above him with the tip of his bike and you will fall off the bike and land on the platform that's directly below the object that you want to get into. Now you're able to move freely inside the object!

CAUTION: Be careful not to move too far from the platform once on it after crashing Wario's bike and ending up in the object! If you do, then you will be out of the object and you will have to go back in by crashing Wario's bike again.

NOTE: This glitch has been tested with ALL and ANY Stage Builder objects except for the few objects that Wario is too large to fit in to or the falling down block! And I am proud to say that all of them work! However, some might be a little harder than others, but they still work!

***I AM THE FOUNDER OF THIS GLITCH AND HAVE NOT POSTED IT ANYWHERE ELSE BUT HERE ON CHEATSGURU!!! If you find this glitch anywhere other than here, PLEASE let me know!***

Feel free to tell me how it works for you in the comment section below! ~

~GamingProdigy~