True, knowing exactly what you're shipping wouldn't be a bad thing.
I assume that the packages are to be sent by a letter, but since I don't know the country of origin, I don't know the allowed dimensions.
Just one idea would be to get a roll of pallet wrap foil or any other packaging material which you crumple up and put inside the package, so that it looks like it is protecting the item, and then hide the stuff inside foil.
To make it look authentic, I would send a original packed article which has a low value of goods.
Send the whole thing as a sample shipment with a fake invoice so that the goods do not have to be cleared through customs and specify a company as the sender and ideally also the recipient.
As already KokosDreams said, make several packages out of it.
Or you could try to create via VPN a Polish label with a Polish shipping address and Polish receiver, which would make it appear that the package was misdirected and so it have passed customs.
Here again it is necessary to hide it as uncovered as possible.
Ideally an article that can be closed again after opening.
For this you would have to do some research yourself, because this method is only known to me by hearsay, so I can't tell you if it will really work that way.
You could first give up a trial shipment, if you still have enough time.
Shipments from non EU countries are unfortunately always associated with some risk