The formulation given in the question is perfectly proper German.
that makes it automatically not proper Bavarian, by definition.
If this is asking for a translation into "Bavarian" it might need to respect the local dialect and not use the High German variant, although it will be understood there probably as well.
The local dialect would sound more like this: Boarische Hozad but possibly using the first word as "bairische".
One variant actually suggested by a translation service then reads
Weaknesses of dubious machine translations aside: With nearly all dialects this spelling is not codified for the whole region of Bavarian dialects. It is not even strictly codified for the Munich variant. There is some leeway in choosing the right letters to include in this short quip. Maybe "vahairod"? (Within the context given the "seit" seems almost optional.)
In my view this is quite a legitimate choice and it will be understood and appreciated.
The conventions for printing stuff on a sports dress make this layout given in the question quite understandable. No real need to change that.
One might opt for a more formal phrase as the basis. That still highlights local language variants and adds a slightly humorous variant. Further it should also satisfie the sticklers that cannot read the usual prints on sports dresses. It might be read as follows:
"20 18 –– Eh gschlossn".