What I meant to suggest by liquidity is that the Roth not be the first account to be tapped, so have some liquidity to avoid Roth withdrawals.
When you take money out of a savings account you don't have to worry about whether it's principle or dividends. You also don't need to be concerned about whether it impacts eligibility for some tax credit or whether you have 30, 60 or 90 days to replace it without issue. And what happens if we have another 2007-2009 bear market where the OPs 4 month EF becomes a 2 month EF because it was invested in a 2040 target retirement fund? So now we have to decide how we take money which is primarily intended for and benefits from long-term tax free growth and invest it to avoid short term losses.
Is it possible to manage those issues? Absolutely. But the OP currently has a $1500 EF. I'm thinking the OP needs a EF management 101 set of guidelines and a lot of the discussion is focusing on issues that come up in the EF Management 201 course. I think his current needs (and hence priorities) should be pretty simple. First, save up 2 months of expenses.
Put it in a savings account, keep it under a mattress or burry it in a jar in the backyard. Anything will do, just save up some money.
Second, once you have some money set aside, start looking at which investment firms you would like to go with and consider opening a Roth. BTW, for many firms, $1500 is a minimum investment, if that. I'm sure others can point the OP to reputable firms that have a low minimum balance, but this is also something to consider. Once you have months 1 and 2 established, then you could consider saving months 3+ in a Roth account.
In the longer term, if the OP really wants to go 100% Roth for the EF, that's his choice, but I think he needs to educate himself on how to manage that and I would start with something simple just to get rolling. That's how I did it and it seems to have worked well for me. There's nothing wrong with going 100% Roth from the start, you just need to maintain good situational awareness and a clear understanding of when and what the money will be used for. For example, our EF also functions as a new car purchase / home improvement / vacation fund. The dollar amounts that we maintain in cash savings tend to vary between 5 and 9 months of expenses. Last summer I used it to fund a new deck and roof with skylight. Once the work was completed, I just wrote the contractor a check for $17K and was done with it. Could I have done that with money from a roth account as well? Sure, but there's probably a few more steps and considerations involved. Maybe I'm just lazy.