Actually, I can strategize a little for Calvinists reading Hebrews. What you have to do is take a "phenomenological" approach. You have to say that not all Hebrews thinks as being "in" are really in. They only appear to be "in" from the author of Hebrews' perspective.
But the language and argument of Hebrews reflects an unambiguously Arminian perspective on the part of the author, "perfectly" expressed in the perfect tense in Hebrews 3:14: "We have become and remain partakers of the Christ if indeed we hold fast the beginning of substance firm until the end."
There is no wiggle room here on what the author is thinking, although again, if I were a Calvinist I would suggest that he is expressing this truth in his own paradigm, not fully understanding how predestination works. But I am not a Calvinist, so I can take what he says at face value.
To partake is not to dabble, any more than Jesus only dabbles with flesh and blood in 2:14. It is to become or assume what you are talking about. Jesus fully became human. These individuals have fully become "Christians."
The perfect tense implies something that was completed in the past. They partook of the Christ and it was done, like someone who gets married, and you are married. It is the remaining married or the remaining partakers that is in question in the verse. The perfect tense implies not only that you were married, a completed act, but that you have remained married ever since. So this verse says that they became a partaker of the Christ and so remained ever since, if indeed they hold fast.
What this verse clearly states, as does the image of the wilderness generation it talks about, is that one's continuance as a partaker of the Christ is contingent on holding fast, on going all the way to Canaan. Those who disbelieved in process, they did not make it, did not persevere to the end.
The perfect tense here is thus "perfect Arminianism," as is the entirety of the book of Hebrews.